1. Field of the Invention
The invention relates to the production of corrosion resistant tubing from welded starting material of metals such as titanium and alloys thereof, having a hexagonal close packed crystal structure at room temperature.
2. Description of Related Art
It is a recognized fact that tubing made by rolling flat stock and welding is less expensive than tubing made by a seamless technique. For instance, it is quite common to use welded tubing for commercial applications such as chemical process tubing which do not require the additional quality that seamless tubing provides. However, there are certain environments where corrosion problems can occur preferentially along the weld seam. This has been observed in titanium as well as in zirconium alloy tubing made for the nuclear industry. These weld seam corrosion problems are due to the large, random grain structure inherent in welded materials. Weld seam corrosion can proceed to the point where the weld seam will fail and open up like a "zipper" under pressure.
A major reason for corrosion problems along the weld seam is due to the formation of metal hydrides. Titanium, zirconium and certain other metals have a susceptibility to hydrogen contamination and under certain circumstances, hydrides form which are by nature very brittle. Cracks which may be present at tube surfaces, will follow along these hydrides when stresses are applied. Therefore, the orientation of the hydrides to the tube wall is very important. If the hydrides are oriented across the tube wall, then there is a very short path for a stress corrosion crack to follow and cause rupture of the tube. However, if the hydrides are oriented in a circumferential direction, then there is no easy path for cracks to follow and no rupture will occur.
It has been shown that the orientation of the metallic crystals determine the orientation of hydrides. Tubing with a "radial" crystallographic texture is oriented such that hydrides are circumferential and do not pose a significant problem. In a welded tube, the base metal may have a radial orientation left over from the strip rolling process. In the weld seam, however, the crystals are very large and random. Some of these large crystals will be oriented in the circumferential direction and hydrides will form within these crystals across the tube wall and cause premature rupture of the tube. This corrosion phenomena is called "delayed hydrogen cracking" (DHC).
U.S. Pat. No. 3,486,219 ("Davies") discloses a method of homogenizing the structure of butt welded tubes useful for nuclear energy applications by cold planetary ball swaging to deform the grain structure and subsequently heat treating to effect recrystallization of the structure. Davies provides examples of preparing tubes of stainless steel and Zircaloy-2. Davies does not disclose making tubes of titanium or titanium alloys.
U.S. Pat. No. 4,765,174 ("Cook") relates to production of tubing of zirconium and alloys thereof. In particular, Cook discloses that it is conventional to subject Zircaloy tubing to multiple pilger reductions and intermediate recrystallization anneals with Q ratios greater than 1, especially in the last or final pilger reduction, in order to produce a textured Zircaloy product resistant to radial hydride formation in service (Column 1, lines 26-68 of Cook). According to Cooks's invention, hot extruded Zircaloy tubing is expanded to enhance radial texturing of the tubing. Cook does not disclose making tubes of titanium or titanium alloys.
U.S. Pat. No. 4,990,305 ("Foster") relates to textured zirconium tubing. In particular, Foster discloses that it is conventional to subject tubing made of zirconium alloys to mechanical and thermal treatments and that pilgering causes the hydrides in the tubing material to be oriented in a circumferential direction (Column 1, lines 14-27 of Foster). According to Foster's patent, tubing is processed in steps to a diameter 10-20% smaller than the final diameter and then subjected to an expansion treatment and anneal to produce a single peak radial texture. Foster does not disclose making tubes of titanium or titanium alloys.
U.S. Pat. No. 4,690,716 ("Sabol") relates to preparation of tubing from a welded precursor tubing of zirconium or titanium. Sabol, however, only provides an example of Zircaloy tubing formed by welding the confronting ends of a rolled sheet together to form a precursor tubing (Column 3, lines 37-40 of Sabol). Sabol discloses a process for producing a homogeneous structure by rapidly heating successive axial segments of the welded tubing completely through the wall to transform the material into the beta phase, rapidly cooling the beta phase tubing, and then subsequently deforming the quenched tubing, by cold working, to produce a final tube (Column 3, lines 52-59 of Sabol). Sabol discloses that the cold working may be effected in a single stage or in a plurality of stages with intermediate recrystallization anneals between each of the plurality of stages and the final size material can be subjected to either a recrystallization or stress relief anneal (Column 4, lines 55-65 of Sabol). Sabol discloses that the cold working may be effected by drawing of the tube or a cold working step, such as pilgering, which will reduce the area of the tubing at least 30% or more (Paragraph bridging columns 4-5 of Sabol). According to Sabol's invention, the precursor welded tubing is heated into the beta phase and quenched in order to produce a homogeneous structure throughout the final tubing (Column 3, lines 42-59 of Sabol).
U.S. Pat. No. 4,717,428 ("Comstock") relates to annealing cold pilgered zirconium base tubing. In particular, Comstock discloses that it is conventional to machine a hollow Zircaloy billet, extrude the billet into an extrusion and subject the extrusion to a number of cold pilger reduction passes with about 50-85% reduction per pass with an alpha recrystallization anneal prior to each pass (Column 1, lines 47-57 of Comstock). Comstock's invention relates to a process for rapid alpha annealing of zirconium based articles rather than the conventional alpha vacuum anneals (Column 4, lines 47-50 of Comstock). Comstock does not disclose making tubes of titanium or titanium alloys.
U.S. Pat. No. 4,728,491 ("Reschke") relates to cladding tube of a zirconium alloy. In particular, Reschke discloses a process of making cladding tubes of a zirconium alloy which are resistant to stress corrosion (Column 1, lines 48-50 of Reschke). Reschke discloses pilger-rolling a starting tube to obtain a cross-section change of the tube wall of 90% or more and produce a finished cladding tube without recrystallization annealing and free of cracks (Column 1, lines 62-66 of Reschke). Reschke discloses that it is advantageous to pilger roll the tube in steps and stress-anneal the tube between two pilger roll passes (Column 2, lines 58-60 of Reschke). Reschke does not disclose making tubes of titanium or titanium alloys.
There is a need in the art for an economical process of making corrosion resistant titanium or titanium alloy tubing from welded stock. Such tubing should possess a homogeneous microstructure with a radial crystallographic texture which is not preferentially attacked by corrosion along the weld seam.